This application is based upon claims the benefit of priority from the prior Japanese Patent Application No. 11-330650 filed Nov. 19, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a signal analyzing apparatus and, in particular, a signal analyzing apparatus adopting the technique for analyzing various kinds analog signals having different band center frequencies and band widths.
In a mobile communications system, for example, a local signal is quadrature modulated with digital data and an obtained digital modulation signal is transmitted and received.
In order to evaluate the apparatus of such a mobile communications system, various analyses are sometimes made on the digital modulation signals of an intermediate frequency band.
FIG. 9 is a block diagram showing a schematic arrangement of a prior art signal analyzing apparatus used in the analysis of a digital modulation signal, etc., of the intermediate frequency band.
In the signal analyzing apparatus 10, first, an input analog signal as an object to be analyzed is converted by an A/D converter 11 to digital data D while being sampled with a sampling signal CKs of a predetermined frequency Fs.
Then the base band components I, Q of the digital data are found by a baseband detection section 12.
Calculation processing is performed by a calculation processing section 13 on the baseband components I, Q to find the characteristic values of the input signal 12.
In the baseband detection section 12, a frequency shift circuit 12a multiplies the digital data D by a local signal of a frequency equal to the band center frequency Fc of the analog signal S to calculate two quadrature signals.
In the baseband detection section 12, digital filters 12b, 12c perform bandpass limitation processing on these quadrature signals and, by doing so, the baseband components I, Q are detected.
In the signal analyzing apparatus thus structured, in order to analyze various signals of different band widths and band centers, it is necessary to use an A/D converter 11 having its operation speed (sampling frequency) correspond to a maximal band width and a baseband detection section 12 having its operation speed correspond thereto.
In recent years, there appears, as a W-CDMA system, a communications system using a digital modulation signal having a band as broad as over 20 MHz.
In order to analyze the signal of such a broadband, it is necessary to use the operation speed of over 40 MHz.
At a present time, as the A/D converter, there has been realized an A/D converter having an operation speed of over 40 MHz.
However, it has been very difficult to realize a baseband detection section for allowing multiplying processing and filtering processing to be performed on digital data outputted from a high-speed A/D converter and for allowing its local frequency and filter factor to freely vary in a broader range.
For this reason it has not been possible to effectively utilize the high-speed characteristic of the A/D converter and to make accurate analysis on digital modulation signals of the W-CDMA system, etc.
It is accordingly the object of the present invention to provide a signal analyzing apparatus which solves the above-mentioned problem and can analyze various kinds of analog signals of different band widths and band centers including a broadband signal such as a digital modulation signal of a W-CDMA system and can do so without sacrificing a high-speed characteristic of the A/D converter.
In order to achieve the above-mentioned object, according to one aspect of the present invention, there is provided a signal analyzing apparatus comprising an analog/digital (A/D) converter (21) for converting a to-be-measured analog signal to digital data while being sampled with a predetermined sampling frequency; a data sorting circuit (22) for sorting the digital data which is outputted from the A/D converter alternately into two signal paths and outputting two digital data each having a xc2xd rate of the sampling frequency in parallel form; a first frequency shift circuit (30) for multiplying the two digital data which are outputted in parallel form from the data sorting circuit by a first local signal whose local frequency is set to xc2xc of the sampling frequency to shift a band center frequency of the digital data signal by xc2xc of the sampling frequency toward a lowband side and generating corresponding two quadrature signals; two xc2xd decimating circuits (31, 32) for receiving the two quadrature signals outputted from the first frequency shift circuit, performing band limitation processing with xc2xc of the sampling frequency as an upper limit and generating two quadrature signals whose rates are reduced to xc2xd relative to input signals; a second frequency shift circuit (33) for multiplying the two quadrature signals which are outputted from the two xc2xd decimating circuit by a second local signal of a pre-set local frequency to shift the band center frequency to a lowband side and outputting corresponding two quadrature signals with the band center frequency set to zero; a controller (40) for setting the second local frequency so as to obtain baseband components of the to-be-measured analog signal as outputs from the second frequency shift circuit; and calculating means (37) for calculating characteristic values of the to-be-measured analog signal by performing calculation processing on the baseband components on the basis from the outputs from the second frequency shift circuit.
According to another aspect of the present invention, there is provided a signal analyzing apparatus, comprising an A/D converter (21) for converting a to-be-measured analog signal to digital data while being sampled with a predetermined sampling frequency; a data sorting apparatus (22) for sorting the digital data which is outputted from the A/D converter alternately into two signal paths and outputting two digital data each having a xc2xd rate of the sampling frequency; a first frequency shift circuit (30) for multiplying the two digital data which are outputted in parallel form from the data sorting circuit by a first local signal of a local frequency set to zero to shift the band center frequencies of the two digital data to zero and generating corresponding two quadrature signals; two xc2xd decimating circuits (31, 32) for receiving the two quadrature signals outputted from the first frequency shift circuit, performing band limitation processing with xc2xc of the sampling frequency as an upper limit and generating two quadrature signals whose rates are reduced to xc2xd relative to input signals; a second frequency shift circuit (33) for multiplying the two quadrature signals which are outputted from the two xc2xd decimating circuits by a second local signal of a pre-set local frequency to shift the band center frequency toward a lowband side and generating corresponding two quadrature signals whose band center frequencies are set to zero; a controller (40) for setting the local frequencies of the first local signal and second local signal so as to obtain baseband components as outputs from the second frequency shift circuit; and calculating means (37) for calculating characteristic values of the to-be-measured analog signal by performing calculation processing on the baseband components on the basis of the outputs from the second frequency shift circuit.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.